[flashrom-gerrit] [S] Change in flashrom[main]: serial: Fix sp_flush_incoming for serprog TCP connections

[Anastasia Klimchuk (Code Review)]

Anastasia Klimchuk has submitted this change. ( https://review.coreboot.org/c/flashrom/+/79112?usp=email )

Change subject: serial: Fix sp_flush_incoming for serprog TCP connections
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serial: Fix sp_flush_incoming for serprog TCP connections

During the development of an esp32 serprog-compatible SPI programmer,
and implementation of the TCP over Wi-Fi for serprog,
I discovered that sp_flush_incoming() silently fails
if the underlying sp_fd descriptor is a TCP socket.

This patch adds a check for this case - tcflush returns ENOTTY,
meaning tcflush is not supported for not terminal objects,
in this case a fallback serialport_read_nonblock loop is used.

TESTED=esp32-serprog, TCP-over-WiFi mode, ~90% connection attempts fail to synchronize without patch; no synchronization issues with patch applied.

Signed-off-by: Stanislav Ponomarev

Change-Id: I9724a2fcd4a41dede2c15f83877efa6c3b0b7fae
Reviewed-on: https://review.coreboot.org/c/flashrom/+/79112
Reviewed-by: Anastasia Klimchuk 
Tested-by: build bot (Jenkins) 
---
M serial.c
1 file changed, 19 insertions(+), 3 deletions(-)

Approvals:
  Anastasia Klimchuk: Looks good to me, approved
  build bot (Jenkins): Verified

diff --git a/serial.c b/serial.c
index 10d739a..577ccf4 100644
--- a/serial.c
+++ b/serial.c
@@ -377,10 +377,26 @@
 #if IS_WINDOWS
    PurgeComm(sp_fd, PURGE_RXCLEAR);
 #else
-	/* FIXME: error handling */
-	tcflush(sp_fd, TCIFLUSH);
+	if (!tcflush(sp_fd, TCIFLUSH))
+		return;
+
+	if (errno == ENOTTY) { // TCP socket case: sp_fd is not a terminal descriptor - tcflush is not supported
+		unsigned char c;
+		int ret;
+
+		do {
+			ret = serialport_read_nonblock(&c, 1, 1, NULL);
+		} while (ret == 0);
+
+		// positive error code indicates no data available immediately - similar to EAGAIN/EWOULDBLOCK
+		//   i.e. all buffered data was read
+		// negative error code indicates a permanent error
+		if (ret < 0)
+			msg_perr("Could not flush serial port incoming buffer: read has failed");
+	} else { // any other errno indicates an unrecoverable sp_fd state
+		msg_perr_strerror("Could not flush serial port incoming buffer: ");
+	}
 #endif
-	return;
 }

int serialport_shutdown(void *data)